Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (9.1)	$ (11.3)	$ (19.6)	$ (17.8)
WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(9.0)	(11.3)	(19.7)	(19.1)
ATC [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(8.8)	(10.9)	(19.5)	(18.7)
ATC [Member] | WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(8.8)	(10.9)	(19.5)	(18.7)
Other [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	(0.3)	(0.4)	(0.1)	0.9
Other [Member] | WPL [Member]
Schedule of Investments [Line Items]
Equity (income) loss from unconsolidated investments, net	$ (0.2)	$ (0.4)	$ (0.2)	$ (0.4)